A directional coupler is a radio frequency device widely used for a signal isolation, a signal separation, etc., and is used to make a radio frequency receiver and a radio frequency transmitter operate at the same frequency band. FIG. 1 illustrates a principle of a directional coupler used in a transceiver system, an input terminal (Input) is connected to an output terminal of a transmitter (TX), and a direct terminal (Direct) is connected to an antenna (Antenna), a coupled terminal (Coupled) is connected to an input terminal of a receiver (RX), an isolated terminal (Isolated) is connected to a matching resistor (Res). There are some definitions for the directional coupler. A loss from the input terminal to the direct terminal is referred as an insertion loss (Insertion Loss), a loss from the input terminal to the coupled terminal is referred as a coupling degree (Coupling), a loss from the direct terminal to the coupled terminal is referred as an isolation degree (Isolation), a difference between the isolation degree to the coupling degree is referred as directivity (Directivity). It is required that the insert loss is very low, the coupling degree is not very large, and the isolation degree is very large, that is, the directivity is strong. For a transceiver system with the same frequency for receiving and transmitting, the energy for transmitting a signal is generally large and a large portion thereof may be coupled to the input terminal of the receiver, however, a receiving signal is very weak and a very strong output coupling signal may affect the useful receiving signal if a directional coupler is not used, which may cause the receiver not to operate properly, as shown in the left part of FIG. 2. After the directional coupler is used, the receiving signal may be partially lost due to the coupling degree, and meanwhile the transmitting signal coupled to the input terminal is greatly attenuated. Thus, the transceiver with the same frequency for receiving and transmitting may operate properly, as shown in the right part of FIG. 2.
Generally, a conventional directional coupler is fabricated by using a transmission line, as described in a reference (Hilal Ezzeddine et al., “Directional coupler”, U.S. patent, U.S. Pat. No. 7,394,333 B2, Jul. 1, 2008). The conventional directional couplers have a large volume, complicated assembling processes and high costs. Then, in order to miniaturize the directional coupler, separated components are used, as described in a reference (Oleksandr Gorbachov, “Directional coupler for RF power detection”, U.S. patent, U.S. Pat. No. 7,576,626 B2, Aug. 18, 2009). However, such a directional coupler is still unable to be integrated. With the rapid development of the integrated circuit, the on-chip integration is a tendency of the future. An integrated directional coupler expands the applicability and has a lower cost, and is applied for, for example, a radio frequency identification (RFID Reader) and so on. In order to implement an on-chip integration for the directional coupler, some special processes are employed, as shown in a reference (Shim, S., Hong, S., “A CMOS Power Amplifier With Integrated-Passive-Device Spiral-Shaped Directional Coupler for Mobile UHF RFID Reader,” Microwave Theory and Techniques, IEEE Transactions on , vol. PP, no. 99, pp. 1, 0). The reference describes a directional coupler fabricated by using an integrated passive device (IPD) process. Though the directional coupler has a small volume, a special processing technology that can not be compatible with a standard CMOS process is used, the cost is high, and a complicated process is used. Thus, it is difficult to achieve an over-chip integration for the directional coupler, and the practical applicability of the directional coupler is limited.
At present, the solution or technology for implementing the directional coupler as described in the known references and patents do not implement the directional coupler by using a standard CMOS process, which greatly limits the application range thereof. For example, in the field of the mobile communication system, miniaturization and portability are highly required. Though some directional couplers have met the requirement of miniaturization, the usage of special process causes a high cost and it is hardly to meet the requirements of the system integration.